Devices that exhibit excessive amplitude and/or gain variation across a frequency band are usually "equalized" to reduce the amplitude or gain variation to less than a few dB over relatively broad frequency bands. For microwave devices a typical range would be from 6-18 GHz. Equalizer circuits, conventionally known as equalizers, are well-known in the art. Examples of such devices are found in H. Tremaine, Audio Cyclopedia .sctn.6(2d ed. 1977). Common RF applications of such equalizer circuits include gain equalization of travelling wave tubes (TWTs), amplitude equalization of radar and communication systems and so on.
Equalizers are either passive or active circuits having a predictable, controlled attenuation slope or characteristic versus frequency. Depending upon the characteristic of the signal they equalize, equalizers typically fall into one of two categories: parabolic or linear. Power tubes such as TWTs generally exhibit parabolic gain shapes where the gain is at a maximum at or near the center operating frequency and at a minimum at the furthermost operating frequencies.
Many conventional equalizers are low voltage standing-wave ratio (VSWR) and absorptive types consisting of a fixed parabolic equalizer and an absorptive fine grain equalizer (FGE). Since the bandwidth range of conventional equalizers is often too narrow to correct the gain variations encountered across the frequency bands of certain devices, for example TWTs, variable hybrid equalizers are sometimes used to equalize the amplitude or gain of such devices.
Hybrid equalizers consist of a main transmission line between an RF input and an RF output, wherein the transmission line has multiple transverse electromagnetic (TEM) mode resonant shunt branches coupled along its length. A fixed equalizer has fixed lengths for each shunt branch or stub tuner with a lossy coating at the end of each stub cavity. By contrast, a variable equalizer has variable stub lengths. By mechanically adjusting the position of a lossy coated plug at the end of a stub cavity, the electrical length and impedance, and hence the loss or equalization profile is altered. For variable equalizers, coverage of up to two and one half (2.5) octaves is common with up to 20 watts of power capability.
Conventional equalizers are designed as separate components that must be individually procured, tested and installed at the system level. Typical dimensions for fixed and variable types of equalizers operating at 6-18 GHz, 5 watts with 15 dB mid-band attenuation, are on the order of 1.25.times.1.0.times.0.5 inches. The equalization of such devices can be typically implemented at any point within a microwave system up to 1 to 5 watts. The disadvantages of conventional equalizers are: large size and weight; high cost; cumbersome handling for insertion into systems; inaccurate (cut and try) design; and incompatibility with multifunctionally designed devices placed in the same environment.
It is, therefore, an object of the present invention to provide an equalizer circuit having desired performance capabilities typical of conventional equalizers, but at a reduced weight, size and cost.